Ritonavir (formerly ABT-538) is a potent protease inhibitor which has shown to represent a major advance in the ongoing battle to combat HIV-1 infection. This is a multicenter Phase III long-term trial to evaluate the safety and efficacy of Ritonavir therapy in subjects who were previously enrolled in one of two studies in which Ritonavir was given alone or in combination with nucleoside analogues. The two earlier studies were terminated by the sponsor, Abbott Laboratories, because their goals were achieved. Virologic response, especially in regard to viral resistance, will be monitored in these patients, who have been treated for a longer period with protease inhibitor therapy.